1. Field of the Invention
The present invention relates to an electrically conductive organic compound. More particularly, the present invention relates to an organic semiconductor that can be used for the production of electronic devices such as transistors and, specifically to an organic semiconductor that can be applied to flexible electronic devices such as electronic paper. The present invention also relates to an electronic device using such an organic semiconductor.
2. Description of Related Art
Performance of liquid crystal displays and organic EL displays has been drastically improved in recent years in the field of electronic displays, and a remarkable progress has been made in high precision and large scale displays. On the other hand, development of displays having an easy-to-see property and flexibility, or shape changeability by bending, and high portability, like the properties of paper as typified by so-called “electronic paper”, has earnestly been required. To accomplish this electronic paper, it is essentially necessary to assemble a thin film transistor (TFT), etc, into a plastic substrate, that is, to accomplish a flexible pixel driving circuit. In existing electronic circuits mainly using polycrystalline silicon (poly-Si) and amorphous silicon (a-Si), however, large-scale processes such as those including application of high-temperature and high-vacuum processes are necessary, and therefore, the application of such a technology is limited to only a small percentage in the production of portable devices and has not yet spread widely, in view of insufficient heat resistance of the plastic substrate used and the production cost. To solve the problems, organic semiconductors that are excellent in flexibility, do not require the high-temperature vacuum process such as vacuum deposition and can be practiced by printing means at a low cost, have drawn increasing attention.
The film formation means of the organic semiconductors can be broadly divided into a group of a vacuum process such as vacuum deposition and a group using a solution such as a spin coating method, a casting method and a printing method. When the organic semiconductors are applied to devices such as field effect transistors, the vacuum deposition method providing high crystallinity of molecules has mainly been employed in the past to accomplish the highest possible carrier mobility. Typical organic semiconductors capable of forming films by the vacuum deposition method are oligothiophene (Applied Physics Letters, H. Akimichi et al, 58(14), Apr. 8, 1991), pentacene (Applied Physics Letters, C. D. Dimitrakopoulos et al, 80(4), Aug. 15, 1996), and copper phthalocyanine. On the other hand, typical organic semiconductors capable of forming films from the solution by the casting method, the spin coating method, or the printing method, are polythienylene vinylene (Applied Physics Letters, H. Fuchigami et al, 63(10), Sep. 6, 1993), and polyalkylthiophene (Applied Physics Letters, A. Tsumura et al, Vol. 149, P1210, 1986, and Journal American Chemical Society, Vol. 1117, p233, 1995). To discover new functions, examples of the film formation method by applying an LB (Langmuir-Blodgett) method for forming a mono-molecular film and for controlling molecular orientation have also been reported (Applied Physics Letters, J. Paloheimo et al, 56(12), Mar. 19, 1990).
When the organic semiconductor described above is used for a channel layer of a field effect transistor, field effect mobility is by far lower than that of an amorphous silicon semiconductor (not more than 1 cm2/Vs) even in the vacuum deposition system for which high mobility has been reported, and is from about 0.1 to about 0.01 cm2/Vs. Further, generally, when the film is formed from the solution, the field effect mobility is further lowered in the order of 1 to 2 figures or location of number because of difficulties in molecular orientation control. In other words, the greatest problem for practical utilization of the organic semiconductors is how to accomplish high field effect mobility through a simple fabrication process.
Generally, the conduction mechanism of the organic semiconductors can be broadly divided into an electric conduction system through the π conjugated system in which the conjugated bonds are distributed inside the molecules, and an electric conduction mechanism through the σ bonds. Polyacetylene and polythiopehene are typical examples of the electrically conductive organic compounds belonging to the former, and polysilane is a typical example of the latter. To obtain an organic semiconductor having high mobility, an organic semiconductor molecular structure which has a large length of the conjugated system chain inside the molecules, and in which activation energy of charge carrier transportation is sufficiently small among the molecules, is suitable. Further, to improve carrier transportation among the molecules, it is effective to orient the organic semiconductor molecules so as to orient the conjugated system in arbitrary directions. As means for controlling the orientation of the organic semiconductor molecules, Japanese Unexamined Patent Publication (Kokai) No. 9-83040 describes an attempt that coats the π conjugated polymer such as polythiophene to an orientation film substrate that is subjected to rubbing treatment, or introduces a liquid crystalline substitution group as the side chain and orients the molecules on the orientation film by external force such as a magnetic field or an electric field.
However, orientation restrictive force by the orientation film is weak in the organic semiconductor molecule not having crystallinity, and sufficient molecular orientation is not accomplished. When the liquid crystalline substitution group is introduced, too, the presence of the substitution groups not contributing to electric conduction lowers the conduction paths of the charge carrier, and mobility drops, on the contrary.